This is a true story about how the nation of Vietnam was named Vietnam.

During The Qing Empire (大清國), Vietnam was a Qing Vassal or Chinese Protectorate. And the Vietnamese King was required to constribute half of his country's GDP, taxation to The Qing Office of Department of Measure in Beijing every year and sent his elite forces to serve in the Qing-Han Army in addition.

The Vietnamese King then decided to name the country as Nam Viet [(南越) in Han-viet} and sent an envoy to Beijing to ask the Qing Emperor to recognise it. But when the envoy presented the official scroll to the Emperor. He felt that if he recognised that name then it would be very easy for him misunderstood with Nan Yue (南越). So he decided to switch the position of 越 before 南(so it became Vietnam [越南]). He then asked the envoy that whether the envoy happy with it or not.

Well i think it's due to Vietnamese grammar different with Chinese. If you say Vietnam, there's no meaning in Vietnamese as it's just a name. But if you say Nam Viet in Vietnamese, (Nan Yue) It means South of the Viet. If Vietnamese is a decedent of Yue, It's as meaningful as Zhong Guo (middle kingdom). However Vietnam (Yue Nan) has meaning in Chinese which mean over the south but not in Vietnamese.There's another reason.. this is just my suggestion that Vietnam wanted to take back Guang Dong and Guang Xi so they suggested that name as all the provinces use to be under one kingdom which was Nan Yue. However the Qin king knew and rejected that name. There's no proof but many times Viet try to ally with Guang Dong and Guan Xi 粤 to push away the Han but not success.